1. Field of the Invention
The present invention relates to a level shifter for converting an input voltage having a predetermined voltage width into an output voltage having a different voltage width, and more particularly to a level shifter for use in a gate line driver of an active matrix display apparatus.
2. Description of Related Art
FIG. 4 is a circuit diagram showing an example of a known level shifter which comprises a first p-channel transistor 51; a second p-channel transistor 52; a first n-channel transistor 54; a second n-channel transistor 55; a positive power supply 56; and a negative power supply 57.
The operation of the circuit shown in FIG. 4 will be described. When an input signal Sig1 is at a low level, an inverted input signal *Sig1 obtained by inversion of the input signal Sig1 is input to the gate of the first p-channel transistor 51 and the first p-channel transistor 51 turns OFF, whereas the second p-channel transistor 52 turns ON because of the input signal Sig1 being input to the gate thereof. Because the positive power supply 56 is connected to an output terminal via the second p-channel transistor 52, the a high level signal Sig2 is output. Also, the positive power supply 56 is connected to the gate of the first n-channel transistor 54 via the second p-channel transistor 52 to turn the first n-channel transistor 54 ON. Through the first n-channel transistor 54, the gate of the second n-channel transistor 55 is connected to the negative power supply 57, and the second n-channel transistor 55 turns OFF.
When an input signal Sig1 is at a high level, on the other hand, the first p-channel transistor 51 turns ON, whereas the second p-channel transistor 52 turns OFF. Accordingly, the second n-channel transistor 55 turns ON via the first p-channel transistor 51, so that the output terminal is connected to the negative power supply 57 via the second n-channel transistor 55, which causes the level of an output signal Sig2 to be low. Further, the gate of the first n-channel transistor 54 is connected to the negative power supply 57 via the second n-channel transistor 55, so that the first n-channel transistor 54 turns OFF.
In a conventional level shifter, a through current flows from the positive power supply 56 toward the negative power supply 57 when the level of an input signal Sig1 changes from low to high, or from high to low, as will be described below. When an input signal Sig1 is at a high level, the states of the respective transistors are as described above. Namely, the first p-channel transistor 51 is ON; the second p-channel transistor 52 is OFF; the first n-channel transistor 54 is OFF; and the second n-channel transistor 55 is ON. At this time, if the level of the input signal Sig1 changes to low, the states of the transistors sequentially change in the following order:
1) First, the first p-channel transistor 51 turns OFF and the second p-channel transistor 52 turns ON.
2) Then, the gate of the first n-channel transistor 54 opens and the first n-channel transistor 54 turns ON.
3) Finally, charges accumulated in the gate of the second n-channel transistor 55 pass through the first n-channel transistor 54 to the negative power supply 57, and the second n-channel transistor 55 turns OFF.
A certain amount of time is required to complete the above change.
Because both the second p-channel transistor 52 and the second n-channel transistor 55 maintain an ON state during the above change, a through current continuously flows from the positive power supply 56 to the negative power supply 57. As a result, such through currents create a problem of high power consumption.
In a level shifter according to the present invention, a single input signal is input to gates of two transistors having different conductivity types, of three transistors connected in series. Accordingly, when the level of an input signal changes, either one of the two transistors which are connected in series necessarily turns OFF, thereby preventing a through current from flowing through the three transistors. As a result, power consumption of a level shifter can be reduced, which further results in an active matrix type display apparatus having a long battery life.
In particular, when an active layer of each transistor is configured of low temperature poly-silicon, the advantage of the present invention can be obtained regardless of mobility of the transistors, thereby achieving particularly notable effects.